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Richard C. Y. Li and Wen Zhou

Abstract

This study examines the changes in tropical cyclone (TC) precipitation and the associated contributing factors over southeast China during 1960–2009. Climatologically, TC rainfall accounts for approximately 20%–40% of the total rainfall over southeast China during boreal summer, and the contribution can even reach 50% for some of the coastal provinces, such as Guangdong, Fujian, Zhejiang, and Hainan. The dominant mode of TC rainfall reveals a dipole pattern over southern southeast China (SSC) and eastern southeast China (ESC), and the associated principal component time series exhibits remarkable interdecadal variations, with two potential change points being identified in the late 1970s and early 1990s. These interdecadal shifts in TC rainfall are also found to be synchronous with two regime shifts in total rainfall, and they can account for more than 40% of the total rainfall anomalies over the coastal regions of southeast China.

To discover the dominant factors responsible for the interdecadal variations, the overall TC rainfall anomalies are broken down into three different components (rainfall frequency, rainfall intensity, and nonlinear terms) based on a new empirical statistical approach. It is found that the interdecadal variation in TC precipitation over SSC is controlled predominantly by changes in TC rainfall intensity as well as TC rainfall frequency, while that over ESC depends mainly on the intensity and the nonlinear terms. Further examination of the TC passage frequency (TPF) suggests that the significant reduction in TPF and TC rainfall frequency over SSC during 1979–92 is associated mainly with suppressed TC genesis (negative genesis effect), while the increase in TPF and TC rainfall frequency during 1993–2009 can be attributed primarily to the enhanced passage probability (positive track effect) over SSC. Meanwhile, variations in TC rainfall intensity seem to be unrelated to the TC’s own intensity change.

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Richard C. Y. Li and Wen Zhou

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This study investigates the intraseasonal variability of tropical cyclones (TCs) by systematically examining the two major components of the intraseasonal oscillation (ISO), the 30–60-day Madden–Julian oscillation (MJO) and the 10–20-day quasi-biweekly oscillation (QBWO). Results suggest that these two ISO modes exhibit different origins, spatial scales, and propagation characteristics, which result in distinctive TC modulation in the western North Pacific Ocean (WNP). The northeastward-propagating MJO predominantly controls the basinwide TC frequency. The significant increase (reduction) in cyclogenesis in the convective (nonconvective) phase is found to be associated with the concomitant strengthening (weakening) of the monsoon trough. In addition, the large contrast in TC frequency also results in a significant difference in daily accumulated cyclone energy (ACE) between the convective and nonconvective MJO phases. The northwestward-propagating QBWO, in contrast, is characterized by alternating signals of positive and negative convection. It leads to the opposite TC modulation in the WNP1 (0°–30°N, 120°–150°E) and WNP2 (0°–30°N, 150°E–180°) regions and results in a northwestward shift in TC genesis locations, which in turn causes substantial differences in intensity distribution and daily ACE for different QBWO phases. Finally, a brief examination of the dual mode situation suggests that the QBWO generally exerts modulation upon the background MJO, and the modulation seems to vary under different MJO conditions.

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Hoffman H. N. Cheung and Wen Zhou

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This study assesses the ability of the 25 GCMs from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to simulate Ural blocking (UB) and its linkage with the East Asian winter climate [December–February (DJF)] in a historical run (1950/51–2004/05). A Ural blocking index (UBI) is defined as the DJF-mean blocking frequency over 45°–90°E for each winter.

Regression analyses suggest that the long-term mean bias of UBI is caused by the long-term mean circulation bias over the North Atlantic. On seasonal time scales, the GCMs simulating a positive bias of UBI are associated with a stronger Atlantic jet stream, as well as stronger westerly momentum fluxes from the North Atlantic to Europe. On synoptic time scales, however, these GCMs tend to be associated with a weaker Siberian high and East Asian trough during the evolution of a UB event. Altogether, there is no apparent linkage between the long-term mean bias of UB and the East Asian winter climate. Further studies are needed to explore the teleconnection between UB and the East Asian winter climate in the GCMs.

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Hoffman H. N. Cheung and Wen Zhou

Abstract

Multiple model ensembles (MMEs) of Ural blocking frequency in 20 CMIP5 GCMs show no apparent increase or decrease in RCP4.5 and 8.5 runs throughout the twenty-first century. However, a significant increasing or decreasing trend of the Ural blocking index (UBI) is identified in individual GCMs, and the trend appears to be correlated with the trend of the Siberian high index (SHI), which measures the East Asian winter climate. Regression analyses reveal that the trend of UBI is related to upstream circulation over the Euro-Atlantic region, such as the intensification of the Atlantic jet stream and the propagation of a quasi-stationary Rossby wave across Eurasia.

In the late twenty-first century, the year-to-year variation of UBI appears to show a stronger linkage with the large-scale circulation over the Kara and Laptev Seas. Meanwhile, UB likely exerts a stronger impact on East Asia on synoptic and seasonal time scales. The uncertainty of UB might present a challenge for accurate prediction of the subseasonal and long-term variation of the East Asian winter climate. To further evaluate the uncertainty in projections of UB, additional work should assess the atmospheric response to the sea surface temperature over the Atlantic and the reduction of sea ice.

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Richard C. Y. Li, Wen Zhou, and Tsz Cheung Lee

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This study examines the climatological features of tropical cyclone (TC) rainfall in Hong Kong in association with different TC-related parameters, and investigates the changes in TC rainfall, non-TC rainfall, and total rainfall during the past few decades in Hong Kong. On average, rainfall induced by TCs can account for about 25% of the total precipitation during summer and fall, and the contribution can be even greater in extreme cases. Composite analysis suggests that extreme TC rainfall is often related to TCs in closer proximity to Hong Kong, with higher intensity, and is associated with stronger convection and moisture convergence in the vicinity of Hong Kong.

Evaluations of the observed trends of different rainfall indices suggest that the rainfall variability in Hong Kong is considerably affected by the TC rainfall, which has a decreasing trend in frequency and intensity in recent decades. Taking out the TC rainfall from the total rainfall reveals that there is an increasing trend in daily rainfall frequency and intensity for non-TC rainfall in Hong Kong. Moreover, time-dependent generalized extreme value analysis of non-TC rainfall also reveals an increase in the return values of the maximum daily rainfall in Hong Kong. Results of this study suggest that, in order to obtain a more comprehensive picture of the long-term rainfall variations in Hong Kong, the contributions of TC rainfall should definitely be taken into account in the analysis.

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Yanlian Zhou, Weimin Ju, Xiaomin Sun, Xuefa Wen, and Dexin Guan

Abstract

Aerodynamic roughness length z om is an important parameter for reliably simulating surface fluxes. It varies with wind speed, atmospheric stratification, terrain, and other factors. However, it is usually considered a constant. It is known that uncertainties in z om result in latent heat flux (LE) simulation errors, since z om links LE with aerodynamic resistance. The effects of z om on sensible heat flux (SH) simulation are usually neglected because there is no direct link between the two. By comparing SH simulations with three types of z om inputs, it is found that allowing z om temporal variation in an SH simulation model significantly improves agreement between simulated and measured SH and also decreases the sensitivity of the SH model to the heat transfer coefficient Ct, which in turn determines the linkage between z om and thermal roughness length z oh.

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Jie Song, Wen Zhou, Xin Wang, and Chongyin Li

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This study investigates linkages between the zonal asymmetry of the annular mode (AM) zonal pattern and the subtropical jet (STJ) over its downstream regions of the storm track by using an idealized model. Observational analyses show that the AM zonal patterns are more zonally asymmetric during days when the STJ downstream of the storm track is unusually strong, and vice versa. In the idealized model, the STJ downstream of the storm track is varied by introducing an additional zonally localized tropical heating. The model’s AM variability exhibits a nearly zonally uniform structure when there is no or only weak tropical heating. However, the signatures of the AM are locally strengthened in the heating sector; thus, the AM zonal pattern is zonally asymmetric when the tropical heating is large enough to create a strong STJ. The model results also show that the percentage of the variance explained by the AM, the persistence of the AM index, and the intensity of eddy feedback are also increased when the tropical heating becomes stronger. It is argued herein that the zonal asymmetry of the AM pattern is caused by the zonal asymmetry of the anomalous synoptic eddy forcing projecting on the AM, which is primarily due to the zonal asymmetry of the variations of the storm track between the nonheating and heating sectors.

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Martin L. M. Wong, Johnny C. L. Chan, and Wen Zhou

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The intensity change of past (1976–2005) tropical cyclones that made landfall along the south China coast (110.5°–117.5°E) is examined in this study using the best-track data from the Hong Kong Observatory. The change in the central pressure deficit (environmental pressure minus central pressure) and maximum surface wind after landfall are found to fit fairly well with an exponential decay model. Of the various potential predictors, the landfall intensity, landward speed, and excess of 850-hPa moist static energy have significant influence on the decay rates. Prediction equations for the exponential decay constants are developed based on these predictors.

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Xiuzhen Li, Wen Zhou, Chongyin Li, and Jie Song

Abstract

The variation in regional precipitation over southeast and southwest China depends strongly on externally imported moisture rather than local evaporation. Associated with the different climate over the two regions, great discrepancies appear in the annual cycles of the moisture supply. Stationary moisture transport dominates externally imported moisture to a large extent, with transient transport being much weaker. The stationary moisture sink over southeast China is strong during spring and summer due to strong moisture input via the southern boundary and weak during fall and winter due to the offset between the output via the southern boundary and the net zonal boundary atmospheric flux. Zonal stationary moisture transport dominates the variation in moisture supply over southwest China. Negative net zonal boundary atmospheric flux countervails (collaborates) with positive meridional transport during the dry (wet) season.

Stationary moisture circulations dominate regional atmospheric moisture convergence anomalies over both southeast and southwest China. Weak cold air activity is favorable for a strong moisture sink over southeast China, while the reverse appears over southwest China in spring. The east-to-west location of the abnormal anticyclone determines whether strong moisture converges over southeast China or southwest China in fall. The anticyclonic circulation anomaly over the Philippine Sea, remotely forced by El Niño, is crucial to the strong moisture sink over southeast China from winter to spring, while it does not play a role in the abnormal moisture sink over southwest China.

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Yeping Lai, Hao Zhou, Jing Yang, Yuming Zeng, and Biyang Wen

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This study compared the efficiencies of two widely used automatic eddy detection algorithms—that is, the winding-angle (WA) method and the vector geometry (VG) method—and investigated the submesoscale eddy properties using surface current observations derived from high-frequency radars (HFRs) in the Taiwan Strait. The results showed that the WA method using the streamline and the VG method based on the streamfunction field have almost the same capacity for identifying eddies, but the former is more competent than the latter in capturing the eddy size. The two algorithms simultaneously identified 1080 submesoscale eddies, with the centers and boundaries determined only by the WA method, and they were further used to investigate the eddy properties. In general, no significant difference was observed between the cyclonic and anticyclonic eddies in terms of radius, life span, and kinematics, as well as the evolution during their life cycles. The typical radius of the eddy in this region was 3–18 km. And a strong correlation was observed between the life span and the radius. The spatial distribution of the eddies indicated that topography played a significant role in the generation of the eddies. And the trajectories of the eddies suggested that all the eddies in this area mostly tended to move southeastward. Statistically, three different stages of the eddy’s life span could be identified by the significant growth and decay of the radius and the mean kinetic energy. This study shows the great capability of HFRs in oceanography research and applications, especially for observing the submesocale dynamics.

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